It sounds to me like something likely involving putting the tea light in the center of the empty pot, surrounding it with water, then lighting it. Then maybe some ritual-type stuff thrown into the instructions, followed by adding the Alkaseltzer or dry ice to the water.

The determination of success will be whether the candle remains lit.

But here's the thing: Dry ice is frozen carbon dioxide. Alkaselzer reacts with water to produce carbon dioxide. CO2 is more dense than the surrounding air,and will pool in the pot. It will deprive the candle of oxygen and extinguish the flame.

If the tell of success is the candle going out, it is nearly guaranteed as long as the air in the room is relatively still (as in you're not too near the air vent or a ceiling fan).